mtd: nand: set correct length to FBCR for a non-full-page write

When we do a non-full-page write, the length be set to FBCR should
not be 'elbc_fcm_ctrl->index', it should be 'elbc_fcm_ctrl->index -
elbc_fcm_ctrl->column'.

Signed-off-by: Liu Shuo <b35362@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>